At uniform speed the acceleration is

Acceleration is defined as the rate of change of velocity with respect to time (a = dv/dt). When an object moves at uniform (constant) speed in a straight line, its velocity does not change with time, so the rate of change of velocity is zero, meaning acceleration is zero. Note: this assumes motion in a straight line; in circular motion at uniform speed, there is centripetal acceleration despite constant speed, but in the context of this basic question, the answer is zero.